Occupying an exposed and fairly immovable position directly on the bone, the nasal mucous membrane is a suitable object of study. When studying the nasal mucous membrane, one occasionally wishes to insert a probe to a carefully determined position in the nose, for instance in order to introduce a chemical substance whose effect on the mucous membrane is to be studied, or in order to carry out a measurement directly on the mucous membrane, for instance measure the circulation in the mucous membrane with the aid of a Doppler laser. In this measurement, the probe has to be placed in a highly exact position in relation to the mucous membrane. The probe must not, for example, press against the mucous membrane, since the pressure might then trigger defence mechanisms altering the circulation. As a result, positioning accuracy in the order of a tenth of a millimeter or so is required. Owing to tremblings of the hand, hair blocking the view and the darkness in the nasal cavity, it is, of course, extremely difficult to manually position a probe so exactly on the nasal mucous membrane. Consequently, previous measurements have not yielded as reliable results as would be desired.